Plasmid-mediated reduced phage sensitivity in Streptococcus lactis KR5.

The phage insensitivity of Streptococcus lactis KR5 was evaluated for its possible linkage to plasmid DNA. This strain possessed plasmids of 40, 29, 26, 21, 16.5, 10.5, 7.8, and 1.5 Mdal. Plasmid curing using novobiocin resulted in derivatives with increased sensitivity to prolate-headed phage, suggesting the involvement of plasmid DNA in phage insensitivity. Transformation of S. lactis LM0230 protoplasts with the KR5 plasmid DNA pool produced transformants containing a plasmid of about 27 Mdal. These erythromycin-resistant transformants were lactose-positive phage-sensitive or were lactose-negative and exhibited a reduced sensitivity to phage. Agarose gel electrophoresis and restriction endonuclease digestion analysis showed the 27-Mdal plasmid band to be composed of two distinct plasmids of 26 Mdal (pBF61) and 29 Mdal (pBF62), which coded for reduced phage sensitivity and lactose-positive phenotypes, respectively. The mechanisms of reduced phage sensitivity encoded by pBF61 included a restriction/modification system and a mechanism that resulted in reduced plaque size independent of incubation temperature. These results further support the involvement of plasmid DNA in the mechanisms for reduced phage sensitivity in dairy streptococci.